Perspectives On Line - Winter 2002: Feature Article / "Zero at the Bone"

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Harold Heatwole grasps the metal handle of the snake-pinner, warily
eyes the four-foot-long copperhead slithering around the nearby terrarium
and glares at the students gathered in the snake shed.

“We do this step by step,”
he patiently explains of the venom-“milking” they are about to attempt.
“We talk to each other at all times to be certain where we are in the
process.”

Seemingly hypnotized like
mongooses before a cobra, the students listen attentively.

After more than 45 years
of handling thousands of snakes in the wilds and in labs, Heatwole has
been bitten only once — by a venomous sea snake in the South Pacific
— and he doesn’t intend for it to happen again, to him or his charges.

The bite was less traumatic
than it could have been because Heatwole, then a zoology professor at
the University of New South Wales, knew what to expect. And that expertise
continually informs his concern for his students’ safety and his research.

Since he came to North
Carolina State University’s College
of Agriculture and Life Sciences in 1991, he and his students have
continued researching the ecological relationships of amphibians and
reptiles. For many years, he has concentrated on sea snakes and other
venomous reptiles, as well as ants and tardigrades, microscopic, four-legged
arthropods that live in water or damp moss.

A professor in the College’s
Department of Zoology,
Heatwole often works under extreme conditions in remote, wide-ranging
locations from deserts to remote islands, including the Sahara, Gobi
and Taklamakan deserts, sites along the Great Barrier Reef, in the Torres
Strait north of Australia, in Central American rainforest canopies and
in Antarctica.

Yet this certified challenger
of the unknown — he’s a fellow of the prestigious Explorers’
Club, which requires a member’s exploits to be documented, is on
the editorial board of the Journal
of Arid Environments, is president of the Australian
Coral Reef and the Australian Herpetological societies and was president
of the Great Barrier Reef Committee — wants to make sure that his students
get credit for their work.

He ticks off a few student
projects, all designed to produce valid experiments, as well as research
experience and training to add to the body of knowledge:

• Bryan Stuart, looking at saliva enzymes in venomous and non-venomous
snakes, found that even a harmless snake’s saliva has a much higher
meat-digesting enzyme evolution than any other vertebrate group.

“Many snakes, if they have
a meal say, late in the season, and can’t raise their body temperatures
to digest it, then their digestion slows so much the meal putrefies,
and they either regurgitate it or it can kill them,” Heatwole says.

“The role of venom in snake
digestive processes indicates that in addition to subduing prey, venom
allows snakes to digest food at lower ambient temperatures than they
otherwise could,” he says. “Perhaps this is why more venomous snakes
occur at higher elevations and latitudes — even above the Arctic Circle
— than non-venomous ones.”

• Adam Parry tracks magnetized mice through non-venomous corn
snakes’ digestive tracts to learn how long it takes them to digest a
meal at different temperatures.

“As expected, the higher
the temperature, the quicker the digestion,” Heatwole says. “So we deduce
the enzymes work faster at higher temperatures. Then we inject mice
with cottonmouth venom — because the venom from the fangs of venomous
snakes might contaminate the specimen — and feed them to harmless snakes
to see the effect venom has on digestibility.

“That experiment is still
in progress, but it looks like at higher temperatures, there is little
difference in the digestion rate of treated or non-treated. But at low
temperatures it looks like digestion is a lot faster, so a venomous
snake is less likely to suffer possibly fatal indigestion than a non-venomous
snake,” he says.

• Former student Mike Bower, sampling seven major cottonmouth
venom toxins year-round at constant temperatures and light, found that
venom changed seasonally, especially in the spring.

“The proportions changed,”
Heatwole says, “but we don’t know why yet, although the hypothesis is
that amphibians are much more abundant in spring and come into the water
to breed, and in cottonmouths and other semiaquatic species, the venom
is specifically tailored and seasonally adjusted to the available prey.
In winter even the volume of venom produced decreases.”

• Jim Green, looking for specific effects of venom in frog tissues
has learned, among other things, that liver is much more venom-sensitive
than muscle.

eatwole,
a native-born American, conducted his ongoing work on snake venom at
Australian universities for more than two decades.

In a recent conversation,
he discussed one aspect his studies, a sort of arms race by creatures
with no arms: the co-evolution of venom resistance by snakes’ prey and
the converse evolutionary toxicity increase by the predators.

“There seems to be almost
an arms race between the highly toxic sea snakes and the eels that are
their prey,” Heatwole notes. “The snakes can swallow an eel almost their
own size. To kill and swallow something that big that can attack you,
you need something strong, and the snakes’ adaptation has been a gradual
development of more virulent toxin over time.

“From the eels’ standpoint,
they are selected to be more resistant to the venom as an adaptation
that allows counterattack and escape. We injected one of those eels
with the equivalent of 52 venom doses, which would have been lethal
to humans, and it barely affected it,” he says.

Then Heatwole found an eel
species on New Guinea reefs that was highly resistant to sea snake toxin
and compared it with others that weren’t a recorded part of the sea
snakes’ diet.

“The ones the sea snakes
didn’t feed on didn’t live in the same habitat, so there had been no
co-evolution, no buildup in resistance to the snake venom; they were
more sensitive to it,” Heatwole says.

Suspecting that the venom-resistant
eel might just be harder to kill, Heatwole and his researchers searched
for comparative eels elsewhere, where no sea snakes live and the eels
have no reason to adapt to venom. Since the venom-resistant Pacific
eel is related to a Caribbean species, they wondered if the Caribbean
eel might also be venom-resistant.

“But when we went to the
Bahamas and tested them out, they were venom-sensitive, so it was co-evolution
in the Pacific,” he says.

Heatwole’s research at N.C.
State also includes a venomous water snake, the cottonmouth moccasin
and its prey.

The deadly cottonmouths
relish fish, frogs, lizards, other snakes, mammals or any vertebrate
but two: bullfrog tadpoles and toads.

“This is almost a reversed
situation from the eels,” Heatwole says. “An animal protected from a
toxic snake by its own skin toxins is interesting. When the bullfrogs
metamorphose, their skin changes and they’re eaten by the cottonmouth,
although when they get larger, their size protects them. But as tadpoles,
they are protected by their skin toxins.

“The toads, on the other
hand, produce skin secretions throughout their lives, and cottonmouths
won’t eat them. They have no immunity to snake venom, but they don’t
need it, because in this case they’re the ones with the toxin. But some
snakes, like the hognose, are resistant to toxic skin secretions from
the toads and eat only toads, having developed a resistance to them,”
he says. “This is also co-evolution.”